In third-generation mobile radio systems, the basic receiver structures used are rake receivers and multiuser detectors. Both concepts allow consideration to be given to multipath propagation of the transmission channel such that as little transmitted signal power as possible remains unused.
The rake receiver involves the use of a finger structure, where each multipath path of the transmission channel is assigned a reception finger of the finger structure. In each finger, one of the multipath components is subjected to a time delay such that the shifted and weighted output signals from the various fingers can be added using the correct phase. This makes it possible to obtain a received signal which is as intensive as possible.
For the purpose of synchronization between the transmission and reception clocks, the received signal in each finger is correlated with the spreading sequence which is known at the receiver end. This is generally done using an “early/late correlator” which correlates the spreading code generated in the received, on the one hand, and the received signal in the finger on the other, for two different sampling phases. To set the optimum sampling time, the sampling time of the received signal is then altered until the correlation value ascertained at the early time and the correlation value ascertained at the late time match.
A prerequisite of this practice is a symmetrical correlation function. Since the signals in the individual fingers respectively describe just one multipath component, however, which means that the sampling time in a particular finger needs to be stipulated merely for one particular multipath component, sufficiently accurate synchronization is obtained by correlation with the spreading code generated at the receiver end.
In the case of the multiuser detector, on the other hand, the complete received signal, including the multipath components, is processed. For this received signal with multipath distortion, the sampling time needs to be stipulated as accurately as possible. Not until a later point in the signal path is channel estimation then performed so as then to equalize the multipath distorted received signal in line with the result of the channel estimation. In the case of the multiuser detector, the equalized received signal is then evaluated such that a plurality of users can be detected simultaneously. Whereas the interference in the signal components transmitted by various subscribers in the same frequency band is suppressed as an unwanted noise term in the case of the rake receiver, the multiuser detector involves evaluation of the received signals from all the subscribers together.
It has been found that, in the case of the multiuser detector, conventional early/late correlation of the received signal with a spreading sequence generated at the receiver end is not suitable for sufficiently accurate stipulation of the optimum sampling time.